The invention pertains to steel and shaped forms thereof which are subjected to extremely rugged service conditions. For convenience of description the invention will be described in terms of die steels and tools, including die blocks and dies, used in forging and related metal shaping operations such as punches, headers and sizing mandrels. As is well known, these applications, including particularly closed die hot work implements, such as press dies and hammer dies, represent some of the most, if not the most, rugged and demanding operating conditions in the entire field of metal shaping and forming.
Referring now specifically to a closed die forging implement such as hammer die, it is axiomatic that such implements must possess high strength and high wear resistance at the elevated temperatures which are encountered in their usual working environments together with great toughness. It is also axiomatic, until this invention, that the properties of high strength and high wear resistance, on the one hand, and high toughness on the other hand, were mutually incompatible to a large extent. Thus, the closed die forger desires high strength and high wear resistance in order to obtain as many thousands of parts from an impression in the die surfaces so that the forging cost per part is minimized to the greatest extent possible. In order to obtain high strength and high wear resistance it has been thought necessary, prior to this invention, that toughness had to be sacrificed to some extent, since the alloy elements which produce high strength and high wear resistance, such as carbon, tend to result in lower toughness. Toughness is used herein in the sense of ductility and deformability under load without cracking.
For applications in which the formed or shaped workpiece has a relatively simple contour such as a link, the sacrifice of toughness in exchange for high strength and high wear resistance, which yields long runs, is not of great concern. However, for applications in which the shaped or formed workpiece is complex in contour, the die steel must have excellent toughness to preclude premature fracture of the tool, and, as mentioned, it has been thought that an increase in toughness is accompanied by a drop in strength and wear resistance, and a consequent decrease in production with its concomitant increase in die cost per piece.
It has been proposed that the ability to have high strength, high wear resistance and excellent toughness can be achieved by the use of appropriate alloying materials, and compositions with these characteristics have been proposed and used. Such compositions however almost invariably have substantial quantities of expensive alloys, such as nickel, and thus the apparent functional advantages derived form the use of alloys is substantially offset by the increased cost with the result that the goal of an economical, high strength, high wear resistance, metal shaping tool with excellent toughness is not available, particularly in those applications which require relatively soft tempered material.
In this connection, and as a frame of reference, tempers will be used in a definitional sense. For ease of understanding in the specifications and claims, the numeric definition of temper levels, in terms of currently widely used industry hardness standards, will be used.
______________________________________ Temper BID BHN Rc ______________________________________ XH (Extra Hard) 2.65-2.75 534-495 54-51 H (Hard) 2.80-2.90 477-444 50-47 1 2.95-3.05 429-401 46-43 2 3.10-3.25 388-352 42-38 3 3.30-3.45 341-311 37-33 4 3.50-3.65 302-277 29-32 Annealed 3.80 255 25 ______________________________________